Method of and means for reducing retroactive currents in threeelectrode ionic amplifiers



Feb. 13, 1934. s. BALLANTINE 1,946,562

METHOD OF AND MEANS FOR REDUCING RETROACTIVE CURRENTS IN THREE-ELECTRODE IONIC AMPLIFIERS Original Filed April 3, 1923 CandacM/rce Imp/1f Conductance. A/aqa/v 1/: q

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wwmwm mkmwt Patented Feb. 13, 1934 UNITED STATES PATENT OFFICE Stuart Ballantine, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application April 3, 1923, Serial No. 629,702 Renewed December 30, 1932 31 Claims.

The invention relates to electrical amplifying circuits and in particular to circuits designed to reduce or to eliminate the retroactive or feedback current in amplifiers of the ionic type. An

object is to provide a method and means for controlling the flow of power in an ionic amplifier stage from the output or plate circuit of the stage to the input circuit thereof through the small capacity between the plate and grid electrodes of the device. A further object is to provide an automatic compensation for the regenerative tendency of this feed-back current in multi-stage amplifiers compounded of a series of ionic tubes linked by impedances.

5; In the accompanying drawing,

Fig. 1 is a diagram of one stage of an ionic amplifying unit,

Fig. 2 is a graphic representation of the variations of input conductance and capacity for varying values of the output impedance,

Fig. 3 is a symbolic representation of one stage of an ionic amplifying unit in which the retroactive eifect may be eliminated by compensation, and

Figs. 4a, 4b, 4c and 4d are diagrams illustrating four different circuits for securing the desired compensation of the retroactive current.

With reference to Fig. 1, the letter T represents an amplifier of the 3-e1ectrode ionic type. An example of this type of amplifier wherein the current is carried solely by negative electrons is the ordinary 3-electrode audion. For convenience of explanation this will be considered as a stage of a cascaded amplifier. When such tubes are cascaded for the purpose of securing a large total amplification a voltage e is impressed across the input terminals of the audion stage, which in this particular case, which is not an embodiment of my invention, coincide with the input terminals of the audion tube and it is customary to connect the succeeding tube across the terminals of an impedance .2, inserted in the plate circuit of the first tube. The voltage drop across this impedance due to the variation of the plate current of the first tube is then available for the excitation of the next tube in the series. The insertion of an impedance in the plate circuit of the device is attended, however, by a flow of current back through the small inherent capacity Cm between grid and plate electrodes, owing to the circumstance of these electrodes being at difierent potentials. The amount of this retroactive current depends among other things upon the character of the 55 plate circuit impedance, and it unfortunate that any adjustment of this impedance to secure the maximum voltage drop across it, for the next tube, cannot fail to increase, at the same time, the retroactive action. The effect of the retroactive current is in general to give the input circuit, even when the grid is so polarized as to permit no ionic grid filament current to flow, a finite impedance which for convenience may be resolved into the reactance of a shunt capacity and a shunt conductance. The first will be referred to as the input capacity, the second as the input conductance. I have been engaged for several years in the study of eifective input impedance in ionic amplifier systems, and in particular have published in the Physical Review, May, 1920, a mathematical expression of the exact dependence of the input constants upon the loadin the plate circuit. For the sake of convenience, I reproduce herewith in Fig. 2, two curves from this paper showing graphically the relation between an inductive plate impedance and the input capacity and conductance in a typical case.

Referring to Fig. 2, in the range AB of plate inductance the input conductance is negative,- the significance of which is that the tube will furnish power to a circuit connected to its input terminals. This range is spoken of as the regenerative range and in particular, as shown by Armstrong (U. S. Patent No. 1,113,149), if an electrical circuit comprising at least one inductance and one capacity is connected across the input terminals it is possible so to adjust the constants of the circuit as to obtain an increase in signal, progressing to a regime of self-sustained oscillation. The energy for this is furnished by the plate batteries of the tube and flows through the plate-grid capacity of the tube, previously referred to. In the range of inductance BC, however, the input conductance is positive and represents an absorption of energy by the tube. The efiective input capacity of the device also undergoes variations as the plate circuit impedance is altered. When the plate impedance is zero the input capacity is simply that of the grid-filament and gridplate electrode capacities in parallel, that is, their sum. In the case of Fig. 2, the input capacity is increased very considerably above this value by the presence of an inductive reactance in the plate circuit. The accompanying changes in the input conductance due to variations in the magnitude of the plate circuit impedance z are a hindrance to the stable operation of amplifying systems and introduce great difiiculty into the design of stable amplifiers. This follows from the fact that the electrical stability of an ionic amplifier whose grid and filament are connected to the coupling impedance of the preceding stage depends, as has been explained, upon the input conductance. If the latter is negative the combination may be unstable. Butv since the amplifying action for which the combination is designed depends upon the constants of the plate impedance in the same way as the regenerative action which produces instability, it is impossible with existing systems so to design the plate impedance to obtain maximum amplification without simultaneously rendering the system so unstable as to be practically useless as an amplifier. It is desirable, therefore, to eliminate the retroaction of the plate circuit upon the input circuit of the stage which causes this instability by compensating the unavoidable;

inter-electrode capacity through which this retroaction occurs. This elimination then produces a complete electrical separation of the output circuit of the stage from the input circuit of the stage and the plate impedance 2 may be designed to produce maximum amplification without a corresponding decrease in stability.

In my study of cascaded amplifier systems I have found that in general the variation of the effective input capacity of the type shown in Fig. 2 is also disadvantageous. Hence the elimination or at least the reduction of the retroactive effects producing variations in the input capacity is also desirable from this point of view. In seeking to eliminate this action I have invented a general method which is always successful and which is also susceptible of wide modification and variation to suit the requirements of the circuit in which the action is to be eliminated. This method and several of its special applications will now be explained.

The method may be stated as consisting in the balancing of portions of the input and output impedance networks in such manner that the fiow of current in the plate circuit will not establish a potential drop across the input circuit terminals. The term input circuit as used in this specification does not refer to the input circuit of the tube (grid-filament terminals) but to the terminals by which the amplifier stage is fed. The tube input terminals will be deseignated as such to distinguish them from the input circuit terminals. In certain cases, however, the input circuit terminals and the tube input terminals may be common. It will be seen that this result can be attained as follows: the potential established at the grid terminal due to retroaction through the intra-electrode capacity may be compensated by making the plate circuit establish the same potential at the other input terminal.

For explaining the general method of balancing the portions of the impedance networks to compensate the retroactive voltage upon the grid the several portions of the impedance network which is symbolically shown in Fig. 3 have been arranged to comprise a Wheatstone bridge (which term is used in its commonly accepted meaning to designate six impedances arranged in trilateral symmetry about a point and interconnecting that point and three others in such a way that the impedances may be represented by the sides and diagonals of a parallelogram). The reduction or suppression of retroactive efiects is efiected by separating the input and output circuits ofthe stage by a method .of bal- .ment conducting branch of the tube is represented by a reactance Xp and resistance Rp in series between P and F, with a sinusoidal voltage e which is impressed in the plate circuit by a corresponding smaller voltage impressed upon the grid. The reactance X will depend upon the character of the ionic conduction through the tube, in particular upon the mobilities and charges of the ions, the geometrical separation of the electrodes, the voltage and the frequency of the current. For a purely electronic conduction at frequencies of the order of 10 with close electrode spacing the internal reactance is negligible; this is the case of the ordinary audion.

An important advantage of my arrangement resides in the fact that no matter what internal impedance the device may possess the compensation of the retroaction by this method is completely independent thereof, that is, it is independent of whether the internal reactive component is present or not. It should be understood that the impedance path PAF together with 6p constitute the electrical equivalent of the internal ionic conducting path of the tube and have nothing to do with the external output circuit. The internal capacity between grid and plate electrodes which causes the undesirable retroaction is shown at Cm. Z0 represents an impedance which may be called the compensating impedance. The other external impedances Z1, Z2 and Z3 are sections of a 3-terminal network having terminals 1, 2 and 3, which may be any arbitrary form. The impedance between terminals 1 and 3 then constitutes the external plate filament impedance of the amplifying system and is identical with the impedance z of Fig. 1.

Now, suppose that for a given value of Cm, the impedance of Z0, and the impedance arms Z1 and Z2 are so proportioned that when current flows from 1 to 3 the ratio of the voltage drop across Z1 to the voltage drop across Cm is equal to the ratio of the voltage drop across Z2 to the voltage drop across Z0. Then the bridge is bal- 125 anced and there is no voltage acting between G and 2; that is, these points are constantly at the same potential regardless of the magnitude of a or of the internal impedance of the tube;

Under these conditions the terminals G and 2 130 may be used as the input terminals of the amplifier stage, and retroaction of the plate filamant circuit (i.'e. of the output circuit) of the amplifier stage upon these input circuit terminals, may be reduced or eliminated. Any form 135 of input circuit, whether tuned or otherwise, may of course be connected between these terminals.

One method of connecting the input circuit to the input terminals of the stage as commonly practiced in this art, comprises using a trans- 14f former, having a primary and a secondary winding, for coupling the amplifier to an appropriate input circuit.

It is convenient in practice so to choose Zc that the balance is independent of frequency in 3145 order to avoid the multiplicity of adjustments that might be necessary in an amplifier designed to operate at a variety of frequencies. This can always 'be done when Cm is a pure capacity, which is substantially true with the ordinary ion audion. To illustrate the wide choice which exists with respect to a selection of the threeterminal impedance and Z0, several specific circuit embodiments are shown in the succeeding figures. The designations are uniform with those of Fig. 3. These arrangements have been chosen to illustrate the method of securing a compensation which can be made independent of the frequency. For this purpose the compensating impedance Zc takes the form of a capacity between the grid and filament terminals, which capacity may be constituted solely by the intra-audion capacity between the elements and incidental distributed capacity or jointly by such capacity and an external capacity.

In Figures 4a, b, c, and d, certain specific embodiments of my invention have been shown. In the modification of Fig. 4a the output circuit of the amplifier stage comprises two capacities, Z1 and Z2, connected in series and shunted by battery B and choke coil L whereby the necessary direct current potential is impressed upon the plate of the audion tube. It will be noted that one terminal, 2, of the input circuit of the stage, is connected to an intermediate point between the two capacities in the output circuit. In Fig. 4b the output circuit of the stage comprises two inductances Z1 and Z2, one terminal, 2, of the input circuit being connected as before to an intermediate point thereof. Fig. 40 represents another modification wherein the output impedance comprises the resistance Z1 and Z2, to an intermediate point of which is connected a terminal, 2, of the input circuit. Fig. 4d shows another modification of my invention in which a terminal, 2, of the input circuit is brought to the alternating current potential of an intermediate point in the output circuit by means of a mutual inductance Z1, between inductances Z3 and Z2, instead of by means of a direct connection such as was used in the previous circuits.

In the specific case in which Z2 is coupled with unity coupling to the lower portion of Z3 the circuit of Fig. 4d becomes electrically equivalent to that of Fig. 41).

It is obvious that in each of the four modifications shown in Fig. 4 the circuit may be balanced to secure a stopping of the retroactive currents from the output terminals, 1 and F, to the input terminals, G and 2, of the stage, in accordance with known conditions of balance for alternating current Wheatstone bridges. With the usual assumption of negligible resistance in the inductive and capacitive arms of any one of these circuits, the fulfillment of the balance condition involves such proportioning of the circuit constants that the algebraic product of the reactances of one pair of opposite non-intersecting arms is equal to the algebraic product of the reactances of the remaining pair of opposite nonintersecting arms. Moreover, it will be seen that the terms involving the frequency in the balance condition for each of the circuits of Fig. 4 cancel out, making the condition of balance mathematically independent of frequency.

While I have described the perfect balancing of the impedance network, according to the principles of the Wheatstone bridge, to secure a complete elimination of the retroactive effect, it will be understood that my invention is not restricted to a perfect balance, since any tendency towards balance with reduction of the retroaction, or control of it, falls within the scope of my invention. It will also be understood that the invention is not limited to the specific circuits which I have described, since various changes or modifications may be made in the circuits without departing from the spirit of my invention.

I claim:

1. Method of preventing retroaction from the output circuit to the input circuit of an audion amplifier .stage of which the impedance elements are arranged as the six arms of an alternating current Wheatstone bridge network of which the two cross arms comprise, respectively, the input and output circuits of said stage, and the four remaining arms comprising impedances each having a certain assigned character, which comprises so arranging the elements of said network that the bridge ,may be balanced independent of frequency throughout the range of frequencies in which said impedances retain their assigned character.

2. An audion amplifier stage comprising an input circuit, an output circuit, and impedance elements each having a certain assigned character cooperating with said input and output circuits to form an alternating current Wheatstone bridge network of which said input and output circuits form conjugate arms; said amplifier stage being characterized by the interconnection of the impedance elements of said bridge network to permit a balance independent of frequency throughout the range of frequencies in which said impedance elements retain their assigned characters.

3. The method of reducing retroaction between the input and output circuits of an audion amplifier stage the impedance network of which is arranged in the form of an alternating current Wheatstone bridge capable of being balanced at radio frequencies, and of which the input and output circuits of the amplifier stage form conjugate arms, which comprises forming each balancing arm of an impedance consisting of a substantially unmixed reactance.

4. The method of reducing retroaction between the input and output circuits of an audion amplifier stage the impedance network of which is arranged in the form of an alternating current Wheatstone bridge capable of being balanced at radio frequencies, and of which the input and output circuits of the amplifier stage form conjugate arms, which comprises making two balancing arms substantially pure capacities and two balancing arms substantially pure inductances.

5. The method of preventing retroaction from the output circuit to the input circuit of an audion amplifier stage having an audion with grid and filament terminals, the elements of said stage being arranged as a Wheatstone bridge capable of being balanced at radio frequencies and having a capacitive arm, which comprises terminating said capacitive arm at said grid and filament terminals.

6. The method of preventing retroaction from the output circuit to the input circuit of an audion amplifier stage having an audion with grid, plate and filament terminals, the elements of said stage being arranged as a Wheatstone bridge capable of being balanced at radio frequencies and which has adjacent capacitive arms, which comprises terminating said capacitive arms respectively at said grid and filament terminals and at said grid and plate terminals.

'7. The method of preventing retroactive effects in an audion amplifier stage having an input circuit, an audion with grid, plate" and filament terminals, and a direct current output path between said plate and filament terminals, which comprises coupling one terminal of said input circuit to said audion grid terminal and coupling the second terminal of said input circuit to a point in said direct current output path at which the alternating current potential is intermediate the potentials of said plate and filament terminals.

8. The method of preventing retroactive effects in an audion amplifier stage having an input circuit, an audion with grid, plate and filament terminals, a capacity external to said audion, and a direct current output path between said plate and filament terminals, which comprises connecting said external capacity across the said grid and filament. terminals, coupling one terminal of said input circuit to said audion grid terminal, and coupling the second terminal of said input circuit to a point in said direct current output path at which the alternating current potential is intermediate the potentials of said plate and filament terminals.

9. The method of preventing retroactive effects in an audion amplifier stage having an input circuit, an audion with grid, plate and filament terminals, and a direct current output path including an inductance between said plate and filament terminals, which comprises coupling one terminal of said input circuit to said audion grid terminal and coupling the second terminal of said input circuit to a point in said inductance at which the alternating current potential is intermediate the potentials of said plate and filament terminals.

10. Method of limiting regeneration in an audion amplifier stage containing an audion tube having plate, grid, and filament elements which comprises causing currents to flow through substantially pure. capacity between grid and filament oi the audion tube, greater than the inherent grid-filament capacity of said tube, and balancing said currents against currents flowing through capacity between grid and plate of said tube.

11. In an audion amplifier stage of the type including an audion having capacity between grid and plate terminals and in which stage the circuit elements are arranged as the arms of a Wheatstone bridge having as conjugate arms thereof the input and output circuits of the stage, the combination of two side arms comprising substantially pure capacities and two side arms comprising substantially pure inductances.

12. The combination with an audion amplifier stage including an input circuit, an output circuit and a three-electrode audion having capacity across the grid and plate terminals and a capacity across the grid and filament terminals, of means establishing compensative currents through said capacity across the grid and filament terminals to oppose retroactive currents through said grid-plate capacity.

13. The combination with an audion amplifier stage including an input circuit, an output circuit and a three-electrode audion having capacity' across the grid and plate terminals and capacity across the grid and filament terminals, of an external capacity across said grid and filament terminals with means establishing compensative currents through said capacity across the grid and filament terminals to oppose filament terminal of said tube, said impedance being inductively. coupled to said plate impedance.

15. In a amplifier stage, the combination with a three-electrode audion, a capacity across the grid and plate terminals of said audion, an input circuit, and an output circuit, of an external capacity across the grid and filament terminals of said audion, and an external inductive coupling between said circuits operative to establish compensative currents through the capacity across the grid and filament terminals to oppose the effects of retroactive currents through said grid-plate capacity.

' 16. In an amplifier stage, the combination with a three-electrode audion, a source of direct current, and a plate impedance comprising a winding having its lower and connected to a filament terminal of said tube through said direct current source and its upper end connected to the plate terminal of said tube, of a second winding wound in the same sense as and coupled to said plate impedance, the lower end of said second winding being connected to a filament terminal and the upper end of said second winding serving as an input terminal for said amplifier stage, the grid terminal of said audion providing the other input terminal for said stage.

I'L'An audion amplifier stage comprising a three-electrode audion, an impedance path between plate and filament, and an input circuit having one terminal connected to the grid of said audion, characte'ized by a conductive connection from the second terminal of said input circuit to the filament of said audion through a portion of said plate filament path having a substantial alternating current impedance at radio frequencies.

18.1111 audion amplifier stage wherein an audion and a plurality of impedances form a Wheatstone bridge network, characterized by the fact that capacity between grid and filament terminals of the audion constitutes one arm of the bridge.

19. An audion amplifier stage wherein an audion and a plurality of impedances form a Wheatstone bridge network, characterized by the fact that capacity between grid and filament terminals of the audion and capacity between grid and plate terminals of the audion constitute adjacent arms of the bridge.

20. An audion amplifier stage having a resonant input circuit, and means for limiting regeneration including an impedance between grid and filament, and connections for mutually balancing against one another currents flowing through saidimpedance and through the gridplate capacity of the audion; said impedance being a substantially pure capacity greater than the inherent grid-filament capacity of the audion.

21. An audion amplifier stage having a resonant input circuit, and means for limiting regeneration, said means including a substantially non-inductive reactance between the grid and filament of the audion, and connections for mutually balancing against one another currents flowing through said reactance and through the grid-plate capacity of the audion; said reactance being smaller than that of the inherent gridfilament capacity of the audion.

22. An audion amplifier stage having a resonant input circuit and means for preventing retroactive effects within the stage, said means comprising capacity including an external element between grid and filament of the audion and connections whereby said capacity is arranged to constitute one arm of an alternating current Wheatstone bridge network.

23. In an amplifier stage comprising a vacuum tube having grid, plate and filament electrodes and capacity between the said grid and plate electrodes and between said grid and filament electrodes, a plurality of serially connected external impedances inserted in the plate filament circuit of the said tube, an input circuit connected to both the grid and a point intermediate the said serially connected external impedances, the said grid filament capacity comprising one arm of a balanced bridge network, whereby retroaction between plate and grid circuits is rendered negligible.

24. An audion amplifier stage of the type having resonant input circuit and an impedance network capable of being balanced to limit regeneration including as an element for balancing the same an impedance between grid and filament, said impedance being a substantially pure capacity.

25. An amplifier stage wherein a vacuum tube having grid, plate and filament, and a plurality of impedances form an alternating current Wheatstone brige having two capacitive balancing arms and two inductive balancing arms, characterized by the fact that said capacitive balancing arms are located between grid and plate, and between grid and filament, respectively, and said inductive balancing arms are serially connected between plate and filament, said grid and the junction of said two inductive arms serving as terminals for the input circuit of said stage, and said plate and filament serving as terminals for the output circuit thereof.

26. An amplifier stage comprising a vacuum tube having grid, plate and filament elements,

and impedances associated therewith arranged as an alternating current Wheatstone bridge of which capacities between grid and plate, and between grid and filament, respectively, form two balancing arms, and of which each of the two remaining balancing arms includes inductance in a direct current path between plate and filament, said plate and filament forming the terminals for an output circuit, and said grid and a point of which the alternating current potential is intermediate the potentials of said plate and filament forming the terminals for an input circuit.

2'7. An audion amplifier stage of the type including a space discharge device, and a plurality of impedances forming an alternating current Wheatstone bridge having two conjugate arms comprising respectively the input circuit of the stage and the output circuit of the stage and four balancing arms, said balancing arms being characterized by the fact that the product of the impedances of one pair of non-adjacent arms is substantially equal to the product of the impedances of the remaining pair of non-adjacent arms throughout the frequency range of the amplifier.

28. An audion amplifier stage as claimed in claim 27 in which each of the four balancing arms is adjacent to one arm having a reactance of the same character.

29. The combination with a three-electrode thermionic device having an input circuit, an impedance connected between a point in said input circuit and the cathode of said thermionic device, to cause between said point and the cathode a substantial difference of potential, a second impedance connected between said point and a point in the anode circuit of said thermionic device, and an input element having its terminals connected, respectively, to said first named point and the grid of said thermionic device.

'30. An audion amplifier stage of the type wherein an audion and a plurality of impedance elements form an alternating current Wheatstone bridge network for opposing the efiects of grid-plate capacity coupling between two circuits associated with the audion, characterized by the fact that said grid-plate capacity forms one balancing arm of the bridge, that a second balancing arm of the bridge is capacitive, and that the impedances of the two remaining balancing arms are resistive, the two said circuits being arranged as conjugate arms of the bridge.

31. An audion amplifier stage wherein an audion and a plurality of impedances form an alternating current Wheatstone bridge network having as conjugate arms thereof the input circuit and output circuit of said stage, characterized by the fact that the two bridge arms terminating at the grid terminal of the input circuit are capacitive, and the impedances of the two bridge arms terminating at the other terminal of the input circuit are resistive.

STUART BALLANTINE. 

